In recent years there has been a significant increase in the use of lasers in medicine. A variety of delivery devices have been developed to couple the light from the laser to the tissue being treated.
One type of delivery device includes a solid, flexible fiber, typically formed from silica. In these devices, the laser light is transmitted along the fiber by total internal reflections off the walls of the fiber.
The other principal type of delivery device consists of a hollow tubular member for channelling the light. The tubular member can be configured as a waveguide with the light reflecting off the internal walls of the tube. Alternatively, the light may be focused in a manner to pass through the tube without substantial contact with the walls.
Hollow delivery devices have certain drawbacks not associated with solid fibers. The most significant drawback is that the internal surfaces of a hollow device can become contaminated by smoke and debris created during a surgical procedure. One common method of reducing this contamination is to direct a flow of purge gas through the device and out the delivery end thereof. By maintaining a positive pressure at the delivery end of the device, it is possible to minimize contamination. As can be appreciated, the use of a purge gas to prevent contamination adds a layer of complexity to the system. Moreover, it is difficult to use such hollow devices under water since the exiting purge gas will create bubbles in the water, disrupting the vision of the surgeon.
A far simpler approach for minimizing contamination would be to seal the end of the hollow device with a window that is transparent to the laser radiation. Unfortunately, this approach has not been easy to implement because it is difficult to design a window which would fulfill all the necessary requirements. More specifically, the window selected must be highly transparent to the laser radiation so that it will not overheat from absorbed laser energy. The window material must also have good thermal conductivity, stability and strength at high temperatures.
The above criteria could all be satisfied by forming the windows from crystalline materials such as sapphire or diamond. These materials are extremely efficient at transferring heat via lattice phonon transmissions. However, these materials tend to have relatively high losses due to reflections at their surfaces. For example, sapphire has a reflection loss in air of approximately eight percent per surface, while diamond has a loss of seventeen percent per surface. As can be appreciated, these losses are too high for practical use.
The common approach for reducing losses due to surface reflections is to coat materials with thin layers of transparent dielectric materials. However, the common dielectric materials used for antireflection coatings are much more fragile than these crystalline compounds and would be easily damaged during use. Thus, it would not be possible to reduce the reflection loss of crystalline windows by using known dielectric coatings in a surgical laser setting.
In the prior art, attempts have been made to use less desirable materials to form a sealed window in a delivery device. For example, in U.S. Pat. No. 4,122,853, a hollow delivery device is disclosed which is sealed with a window formed from zinc selenide. Unfortunately, the properties of zinc selenide, such as its melting point, hardness and thermal conductivity, make zinc selenide a poor choice for all but the lowest laser power levels. There is also a concern that zinc selenide may be toxic and therefore it is a far less desirable material than inert crystalline materials.
Accordingly, it is an object of the subject invention to provide a hollow delivery device having a sealed window which overcomes the problems found in the prior art.
It is a further object of the subject invention to provide a window for a hollow delivery device which is formed from a durable, inert, crystalline material.
It is still another object of the subject invention to provide a sealed window for a hollow delivery device which is configured to reduce the reflection losses at a selected wavelength.
It is still a further object of the subject invention to provide a sealed window for a hollow delivery device which is configured to reduce the reflection losses at a selected wavelength without using dielectric coatings.
It is still another object of the subject invention to provide a sealed window for a hollow delivery device which is configured to reduce the reflection losses at a two different selected wavelengths without using dielectric coatings.
It is still a further object of the subject invention to provide a sealed window for a hollow delivery device which is configured to reduce the reflection losses at 10.6 and 11.1 microns without using dielectric coatings.